This invention relates to inertial instrument sensors. In particular, the present invention is a ring laser gyroscope dither spring structure and method of manufacture of a dither spring that minimizes thermally induced stress effects on the ring laser gyroscope block.
A ring laser gyroscope (RLG) is commonly used to measure the angular rotation of a vehicle, such as an aircraft. Such a gyroscope has two counter-rotating laser light beams which move within a closed loop optical path or "ring" with the aid of successive reflections from multiple mirrors. The closed path is defined by an optical cavity which is interior to a gyroscope frame or "block". In one type of RLG, the block includes planar top and bottom surfaces that are bordered by six planar sides that form a hexagon shaped perimeter. Three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the corners of the optical path which is triangular in shape.
Operationally, upon rotation of the RLG about its input axis (which is perpendicular to and at the center of the planar top and bottom surfaces of the block), the effective path length of each counter-rotating laser light beam changes and a frequency differential is produced between the beams that is nominally proportional to angular rate. This differential is then measured by signal processing electronics to determine the angular rotation of the vehicle.
Because of backscatter radiation, which is created as the laser light beams are reflected at the mirror surfaces, and other factors, the frequency difference between the counter-rotating laser light beams disappears when the angular velocity of the RLG about its input axis has a value that is below a particular threshold. This phenomenon is called "lock-in", and the range of angular rotation over which lock-in occurs is the "deadband" of the RLG. This phenomenon is undesirable because, at low rotation rates, lock-in produces an indication that no rotation is occurring when in fact, there is low rate angular rotation. Therefore, any inability to accurately measure low angular rotation rates reduces the effectiveness of the RLG in vehicle navigation systems.
There are several known approaches to eliminating the lock-in phenomenon. One such approach involves using a drive motor for mechanically oscillating the RLG about its input axis so that the RLG is constantly sweeping through the deadband and is never locked therein. This mechanical oscillation of the RLG is called dithering. Dithering is accomplished by mounting gyroscope block on a flexure device known as a "dither spring".
One such known dither spring 10 for dithering a gyroscope block 12 of a RLG 14 is illustrated in FIG. 1. The dither spring 10 is generally composed of a central member or hub 16 (which is centered on the input axis 18 of the RLG 14) having a plurality of flexible radial members or reeds 20 extending between the hub 16 and a continuous toroidal rim 22. Triangular shaped lobes 24 extend outwardly from the hub 16, and one of the lobes 24 is interposed between each pair of adjacent reeds 20. Each lobe 24 has a fastener aperture 26 for appropriate fasteners, such as bolts (not shown), to fasten and secure the dither spring 10 to an appropriate gyro support or inertial platform (not shown). The toroidal rim 22 is defined by arcuately spaced mounting sections 28 through which the dither spring 10 is secured within the block bore 30 of the gyroscope block 12 by a suitable adhesive. The mounting sections 28 are arcuately spaced by way of rim notches 32 located at the ends of the reeds 20. Each reed has a pair of piezoelectric transducers (PZT's) 34 mounted on opposite sides thereof via a suitable adhesive. The combination of the dither spring 10 and PZT's 34 defines drive motor 11 for mechanically oscillating the RLG 14 about its input axis 18.
Voltages are applied to the PZT's 34 such that one PZT on each reed 20 increases in length while the other PZT decreases in length. The effect of these length changes in the PZT's 34 is transmitted to the reeds 20 through the mounting of the PZT's 34 thereon. Increasing the length of one side of each reed 20 while shortening the other side causes the reeds 20 to flex or bend so that each reed 20 experiences a small rotation about the RLG input axis 18. The voltage is oscillatory so that the reeds 20 are constantly vibrating in phase and the gyroscope block 12 mounted to the toroidal rim 22 rotates about the input axis 18. The amplitude of the dithering is generally carefully controlled and monitored to minimize the effects of lock-in. Since the dither oscillation angular velocity and displacement can be constantly monitored, they can be excluded from the output signal of the RLG 14.
Though the above described, known dither spring 10 adequately dithers the gyroscope block 12 of a RLG 14 so as to prevent lock-in, there are some disadvantages. Currently, the dither spring 10 illustrated in FIG. 1, is machined to shape from a dither spring "blank" via a process known as electro-discharge machining (EDM). EDM removes material through the use of an electric spark generated by a high energy power supply. To form the known dither spring 10 of FIG. 1, requires that the dither spring "blank" be first drilled to form a central locating aperture (which also defines the RLG input axis 18) for the EDM cutting element. Next, the three, lobe fastener apertures 26 are drilled. Subsequently, an EDM cutting element is setup three times to remove material from the "blank" (see reference numerals 36, 38 and 40) to form the three lobes 24 and three reeds 20 of the dither spring 10. Finally, the EDM cutting element is setup a fourth time to remove material from the blank (see reference numeral 42) to form the outer periphery of the toroidal rim 22. Due to the multiple drilling steps and multiple EDM cutting element setups required to machine the dither spring 10 to final form, the known dither spring structure shown in FIG. 1, is costly and time consuming to manufacture.
In addition to the dither spring manufacturing disadvantages described above, a RLG incorporating the known dither spring 10 shown in FIG. I is sensitive to temperature changes because of the different rates of thermal expansion and contraction of the gyroscope block 12 and the dither spring 10 (due to the differences in the coefficients of thermal expansion (CTE's) of the materials from which the block 12 and dither spring 10 are manufactured). In practice, though the block 12 is generally temperature stable, the dither spring 10 experiences thermally induced dimensional changes. These dither spring dimensional changes are transferred to the block 12 via the block bore 30 imparting mechanical stress to the block 12, thereby degrading the accuracy of the angular rotation data output of the RLG 14 and the lifetime expectancy of the dither spring 10 and the RLG 14. Though voids 44 (see FIG. 1) between the toroidal rim 22 and the block bore 30 (created by the rim notches 32) allow substantially stress free, radial length expansion and contraction of the reeds 20 due to temperature changes, the known dither spring 10, does not readily permit stress free circumferential expansion and contraction length changes of the rim 22 under temperature changes.
Hence, there is a need for an improved dither spring structure and method of dither spring manufacture. In particular, there is a need for a RLG dither spring structure that can expand and contract under temperature changes while minimizing the mechanical stress imparted to the gyroscope block of the RLG. In addition there is a need for a dither spring structure that is less costly and time consuming to manufacture.